1. Field of the Invention
The invention relates to a device for monitoring the sealing properties of a gas-filled chamber in connection with a pressure measurement device for measuring the gas pressure present in the chamber.
2. Brief Description of the Background of the Invention Including Prior Art
Such a device is known from the European Patent EP-B-0,248,821. This known device serves for monitoring the sealing properties of a vehicle tire and, in this sense, the device scans the tire pressure with a pressure measurement device. The measurement value resulting from the signal of the pressure measurement device is evaluated with the aid of a switching circuit. The switching circuit exhibits a switching threshold corresponding to the nominal tire pressure such that, in case of a falling below the nominal tire pressure, a signal is generated with the switching device.
Since the tire is subject to temperature variations, the tire pressure changes with these temperature variations. As a consequence, the preset switching threshold value deviates more or less from the nominal tire pressure depending on the temperature level. Therefore, the pressure measurement device is furnished with a temperature-sensitive device, which can be actuated depending on the tire pressure and which serves to compensate the switching threshold to the nominal tire pressure changing with temperature level.
A reference pressure box, closed on all sides, serves in this case as a temperature-sensitive device. The reference pressure box is filled with a medium. The pressure of the medium is to correspond to the nominal tire pressure. The reference pressure box is surrounded by the tire temperature and can expand or contract according to the variations in the tire temperature. This change in shape of the reference pressure box is employed to influence the pretensioning of a spring-loaded switching device such that the switching threshold value is adapted to the temperature-dependent pressure change of the nominal tire pressure.
However, this device is only functioning as long as no leak occurs at the reference pressure box. In case of such a leak at the reference pressure box, there would occur a pressure exchange and balancing between the tire and the reference pressure box.
In case of a leaking reference pressure box, the pressure balancing occurs between the tire and the reference pressure box and the pressure-sensitive switching device is actuated and consequently a signal is generated. This signal, however, is not clearly identifiable because either a tire pressure below a recommended limit or a leakage of the reference pressure box is indicated thereby.
A requirement that the reference pressure box remains ready for operation over a relatively long operating time without the recited functional interference, can only presuppose a very expensive production method. Given the present doubts regarding operational safety, there would in addition be required repeated leakage controls or an expensive monitoring at the reference pressure box.
A pressure measurement device is known from the German Patent DE-PS 2,813,058-C2, where a pressure-sensitive device is formed by a spring-loaded piston. The piston can receive the tire pressure against the force of the spring. The pretensioning of the spring is set relative to the nominal tire pressure such that, in case of a pressure drop below a recommended limit, the spring force prevails and the piston is moved into a signal-generating position. Since this device does not exhibit any means to which the tire pressure can be applied, which means would solely serve to the recognition and the evaluation of the tire gas temperature, the leakage problem described in the European Patent EP-B-0,248,821 cannot occur in this case. Instead, however, the disadvantage has to be accepted that a switching threshold, once fixed by the spring force, cannot be adapted to the nominal tire pressure since this nominal tire pressure is in fact a falsified tire pressure due to variations caused by temperature interaction.
The German Patent Application Laid Out DE-AS 1,183,328 to Jack Kingsley Gould teaches a thermostatically controlled valve responding quickly to temperature changes. The valve is furnished for limiting the temperature of a liquid, wherein a temperature limit can be reached within a relatively short time. As the present invention is directed to the sealing of a gas-filled chamber, it appears that this reference only tangentially concerns the invention device.
The German Patent DE-PS 1,106,095 to Hubert Jung teaches an apparatus for testing the seal based on a temperature-compensated difference pressure determination. The reference shows bellows-type structures in the sketches, however, no threshold control of the gas density appears to be taught in this reference.
The German Patent Application Laid Open DE-OS 2,941,343-A1 to Rudolf Vollmer teaches a thermostatic valve and, in particular, a thermostatic heating body valve. The reference requires an additional working element subjected to a heating agent, where the operating motion, in case of a parallel temperature change, is opposite to that of ambient air work element. It appears that this reference thermostatic valve does not operate to furnish a threshold value for a continuous gas-density-level threshold control in a gas chamber.
The German Patent Application Laid Open DE-OS 2,930,340 to Heinz Schmidl et al. teaches a method for testing the sealing capabilities of tanks. The reference employs temperature measurement apparatus coordinated to each container and feeding of reference values therefrom to an electronic computer for processing and followed by a comparison of the calculated values. It appears that this control system does not teach the automatic actuation of a valve upon passage of a gas density in a pressure chamber below a predetermined gas density level.
The German Patent Application Laid Open DE-OS 2,847,119 to William Barrie Hart teaches a temperature-sensitive control device. The reference teaches that a control element is moved, in case of a temperature change, within the transition temperature region along a path and that devices are furnished in order to select a part of the transition temperature region for a certain region of shift values of the control element. Furthermore, an adjustment means is furnished in order to set that temperature within a selected part of the transition temperature region, where the control element is disposed in a predetermined control position. However, this reference does not provide a threshold control, where the threshold level is determined by the gas density independent of temperature variations.
The German Patent Application Laid Open DE-OS 2,436,812 to Georgi Cwetanski teaches an encapsulated bimetal device. The reference shows in FIGS. 1 and 3 the structure bimetallic disks, which can expand based on temperature changes.
The German Patent Application Laid Open DE-OS 2,358,649 to Mario Posnansky teaches a measurement device influenced by interfering ambient-temperature changes. The structure taught in the reference comprises a compensation member, which changes the size of a volume-containing medium under the influence of an interfering temperature change, where the dimensions and the thermal expansion number of the compensation member are selected under consideration of the thermal expansion number and under consideration of the medium comprised in the predetermined space such that the influence of interfering temperature changes on the device is compensated. It appears however that this reference does not teach a device which is capable of setting a gas density threshold level for actuating a valve based on a change in gas density independent of a temperature change.
The German Patent Application Laid Open DE-OS 2,111,968 to Costantino Broetto teaches a quick reacting bimetal thermostat. The reference teaches in FIG. 2 a disk-shaped bimetal element 7, which will change its shape based on temperature changes.
The French Patent 2,098,959 to Edmond Ventre teaches a motor means based on thermal effect. FIG. 3 of this reference illustrates a helical metallic part made up of two rods 20, 22, which will change its configuration based on temperature changes. However, this reference is clearly not addressed to the problem of setting a constant threshold level for monitoring the gas density in a gas chamber.
The German Patent Application Laid Open DE-OS 2,856,313 to Marc Deman teaches a thermostatic element. This element includes an expandable material on a wax basis maintained in a chamber. Reference to a threshold control for the density of a gas is not present in this reference.